Re: [PATCH 0/2] arm: add kvm-psci-version vcpu property

[Sebastian Ott <sebott@redhat.com>]

On Thu, 11 Sep 2025, Peter Maydell wrote:
> On Thu, 11 Sept 2025 at 15:49, Sebastian Ott <sebott@redhat.com> wrote:
>>
>> This series adds a vcpu knob to request a specific PSCI version
>> from KVM via the KVM_REG_ARM_PSCI_VERSION FW register.
>>
>> Note: in order to support PSCI v0.1 we need to drop vcpu
>> initialization with KVM_CAP_ARM_PSCI_0_2 in that case.
>> Alternatively we could limit support to versions >=0.2 .
>>
>> Sebastian Ott (2):
>>   target/arm/kvm: add constants for new PSCI versions
>>   target/arm/kvm: add kvm-psci-version vcpu property
>
> Could we have some rationale, please? What's the use case
> where you might need to specify a particular PSCI version?

The use case is migrating between different host kernel versions.
Per default the kernel reports the latest PSCI version in the
KVM_REG_ARM_PSCI_VERSION register (for KVM_CAP_ARM_PSCI_0_2) -
when that differs between source and target a migration will fail.

This property allows to request a PSCI version that is supported by
both sides. Specifically I want to support migration between host
kernels with and without the following Linux commit:
 	8be82d536a9f KVM: arm64: Add support for PSCI v1.2 and v1.3

Regards,
Sebastian